There are problems related to having multiple telephone jacks connected to each other in series (cascaded) or parallel for residential subscribers and the like. An incoming telephone line is connected to a first main jack with a chain of slave jacks connected to it. In some cases all jacks have telephones connected to them. Hence, an incoming call will make all the telephones ring. A person in another room than a receiving off-hook telephone could listen to a conversation without the knowledge of a person who receives the call, which is a kind of eavesdropping.
A more up to date problem concerns that jacks do not only have telephones connected. Other connected devices could be facsimile (fax) machines, modems or the like. When there are multiple connected jacks with all kinds of machines connected such as a fax, it is inconvenient if a person hears a ring signal intended for the fax and believes that it is a phone call and answers the call.
If a telephone is used without knowledge of an ongoing fax call on the line it could disturb or brake the fax transmission depending on how jacks are coupled in a chain and where the off-hook telephone and/or fax is situated in a chain of connected telephone jacks.
In a plurality of countries the jacks for residential subscribers are connected so that every jack is provided with a unique priority. By priority is understood that the jack having the highest priority will break all communication to other telephone devices when off-hooked. A telephone that has the next highest priority will break the connection with all devices having a lower priority then the next highest etc.
To be able to speak in a telephone only two wires (one telephone line) are needed. To every jack in a resident there are two incoming wires to a jack and two outgoing wires. In the jack having the highest priority, the incoming wires are connected to the PSTN (Public Switched Telephone Network) and the outgoing wires to a telephone with the next highest priority and so forth. This creates a chain where every telephone is able to break the chain at off-hook state, thus disconnecting telephone sets with a lower priority.
When no telephone is plugged-in to a jack, the incoming wires and outgoing wires are connected to each other. A plug-in of a telephone will break the chain, whereby the telephone will control break and contact in the chain. Conventionally when a telephone is on-hook a signal will pass through it. When the handset is lifted to an of-hook state the connection between incoming wires and outgoing is interrupted. Hence, disconnecting every telephone with a lower priority.
Today jacks have four connecting terminals and one terminal for an interrupting function. Two of the terminals are used to connect to a telephone, The other two are used to connect to the next telephone in the chain. As mentioned, when no telephone set plug is inserted in a jack, the four terminals are short-circuited. The terminals are divided by the plastic center plug in the connector plug for the telephone when inserted.
If multiple telephonic devices are plugged-in to the same jack, a plug with a body is used which one end is adapted to be inserted into a standard wall telephone jack, and having at least one receiving socket at another end, adapted to receive a conventional plug which is attached to a telephone line operated device.
In some connections, fax machines or modems only work properly if they are connected to the incoming telephone line jack.
When an analogue telephone system is made digital, for example, ISDN, there are still analogue outputs connected that can be used to connect old analogue equipment such as fax, telephone set, modem etc. The present invention provides a smooth change from a analogue telephone system to a digital one.
The U.S. Pat. No. 5,231,658 by Eftechiou discloses a telephone privacy device with a plug-in body for a multiple jack telephone line. Privacy is accomplished through off-hook detection and there is no signaling or control between plug-in bodies via the telephone line.
PCT application WO 92/15 167 discloses a telephone adapter for telephone sets which accomplishes privacy through off-hook detection, and uses a kind of audio signaling.
Patent application document EP-A2-0348 956 describes how privacy is reached by of-hook, lifted line handset, detection.
An off-hook detection for privacy is also disclosed through PCT application WO 95/18502.